1. Field of Invention
The present invention is in the field of discrete power semiconductor devices in general and discrete power diodes in particular.
2. Prior Art
Power diodes of the type this invention is concerned with are generally known and are used for various applications. In order to attenuate overvoltages which occur in the commutation process, i.e. in the reversal of the direction of the current in the diode, power diodes are provided with protective circuits which mostly consist of an RC combination.
In the commutation of the load current, destructive overvoltages can occur in such circuits which cannot be prevented even with the above-mentioned RC circuits. This is due to the fact that the protective circuits themselves are inductive even though slightly, so that an additional voltage occurs therein which corresponds to the decrease of the current in the diode.
Investigations have shown that one must distinguish essentially between conditions in connection with the commutation of the current in a power diode. These conditions are illustrated in FIG. 1. The current pattern can be divided into two partial ranges, the first one of which extends in time up to the point t.sub.0. Up to the point t.sub.0, the current initially drops from a stationary value steadily towards zero. A reverse current then flows up to the point in time t.sub.0, designated A in FIG. 1, which is essentially caused by the reduction of those charge carriers in the middle zone of the diode which have flooded the middle zone in the forward direction. At the point in time t.sub.0, the number of free charge carriers has reached the doping concentration at a boundary to one of the higher doped zones. Following the point in time t.sub.0, a potential on the power diode builds up, whereby a spacecharge zone is formed, starting in almost all cases from the pn junction.
The potential which then builds up on the diode opposes the EMF of the circuit and results in a decrease of the reverse current which is caused by the driving out of the carriers, denoted as B in FIG. 1. In this case, the current curve may, as shown by curve i.sub.1, drop to zero gradually or it may as shown by curve i.sub.2, drop to zero rapidly, i.e. it can practically collapse.
The illustrated different current patterns produce, together with the inductances in the circuit, qualitatively and quantitatively distinct potential patterns on the diode. While, with a gradual attenuation of the current i.sub.1, the voltage U.sub.1 overshoots only relatively little before reaching the EMF, the abruptly falling current i.sub.2 causes voltage U.sub.2 to overshoot considerably. This can result in the destruction of the diode.